1. Field of the Invention
The present invention generally relates to driving light-emitting diode (LED), and more particularly to system and method for driving the LED with high efficiency in power consumption.
2. Description of the Prior Art
The light-emitting diode (LED) is widely used in a variety of electronic devices for diverse purposes. For example, the LEDs may be utilized in the backlight module of a liquid crystal display (LCD) to provide backlight, or may provide flash light in a charge-couple device (CCD) camera. In practice, the LED is, however, temperature dependent, or, in other words, the characteristics of the LED vary according to its temperature. FIG. 1 shows an LED and its equivalent circuit. The LED is equivalently made of a voltage source connected in series with a negative-temperature-coefficient (NTC) resistor. The resistance of the NTC resistor falls with increasing temperature caused by the current flowing through the LED, and vice versa. Accordingly, the voltage potential across the anode and the cathode electrode of the LED (or the forward voltage VF) decreases with increasing temperature by a constant current flowing through, and vice versa.
There are two conventional methods for driving the LED or LEDs: the constant-voltage driving method and the constant-current driving method. In the conventional constant-voltage driving method, the anode electrode of the LED controllably receives a constant-voltage supply. As discussed above, the current flowing through the LED will vary even though the anode electrode receives the constant voltage. Consequently, the LED suffers varying driving current, and thus its associated illuminance. Furthermore, the LED in the conventional constant-voltage driving method is typically connected in series with a current-limiting resistor, which disadvantageously consumes precious power.
In the conventional constant-current driving method, the driving current through the LED is controllably constant. Although the LED driving current (and its associated illuminance) in the conventional constant-current method does not vary with respect to the fluctuating forward voltage VF, the LED, however, is connected in series with a current-sensing resistor, which disadvantageously consumes precious power.
For the foregoing reasons that either conventional constant-voltage or constant-current driving method wastefully consumes power, a need has arisen to propose a novel driving scheme with increased efficiency in power consumption, while maintaining constant driving current.